1. Field of the Invention
The present invention relates to magnet powders and isotropic rare-earth bonded magnets.
2. Description of the Prior Art
For reduction in size of motors or other electrical devices, it is desirable to have a high magnetic flux density of a magnet when it is used in the motor or the like with the actual permeance. Factors determining the magnetic flux density of a bonded magnet employed in an electrical device are the magnetic performance (the magnetization, in particular) of the magnet powder used and the content (compositional ratio) of the magnet powder in the bonded magnet. Accordingly, when the magnetic performance (magnetization) of the magnet powder itself is not sufficiently high, a desired magnetic flux density cannot be obtained unless the content of the magnet powder in the bonded magnet is raised to an extremely high level.
At present, majority of high performance rare-earth bonded magnets in practical use are the isotropic bonded magnets made by using MQP-B powder manufactured by MQI Corp. as the rare-earth magnet powder. The isotropic bonded magnets are superior to the anisotropic bonded magnets in the following respect; namely, in the manufacture of the bonded magnet, the manufacturing process can be simplified because no magnetic field orientation is required, and as a result, the rise in the manufacturing cost can be restrained. However, the conventional isotropic bonded magnets represented by those manufactured by using MQP-B powder have the following disadvantages.
(1) With the conventional isotropic bonded magnets, it is not possible to secure a sufficiently high magnetic flux density. In other words, because of the insufficient magnetization of the magnet powder used, the content of the magnet powder in the bonded magnet has to be raised. However, the increase in the content of the magnet powder leads to the deterioration in the moldability of the bonded magnet, so there is a certain limit in this attempt. Moreover, even if the content of the magnet powder is somehow managed to be increased by manipulating the molding conditions or the like, there still exists a limit to the magnetic flux density obtainable, which stands in the way to the reduction of the size of the motor.
(2) Because of the high coercivity of the magnet powder, it has a poor magnetizability and requires a relatively high magnetic field for magnetization.
(3) Although there are reports on nanocomposite magnets having high remanent magnetic flux densities, their coercive forces, on the contrary, are so small that the magnetic flux densities (for the permeance in the actual use) obtainable for the practical motors are very low.
If the content of the conventional magnet powder in the bonded magnet is increased in order to enhance the magnetic flux density up to the upper limit (that is, if the density of the bonded magnet is made extremely high), then the following problems will show themselves up.
a) In the normal method of manufacture, either molding becomes difficult or the moldability is deteriorated. Because of this, there arise such necessities of, for example, limiting the molding method to the compression molding, raising the pressure and the temperature required for the molding, increasing the scale of the molding machine or introducing a specialized apparatus, or imposing strict restrictions on the type and composition for the binding resin to be used.
b) Reduction in the dimensional accuracy of the bonded magnet obtained due to deterioration in the moldability.
c) Reduction in the corrosion resistance and heat resistance of the bonded magnet obtained.
d) Brittleness of the bonded magnet obtained, which makes cracks and chippings liable to occur, resulting in the lack of sufficient mechanical strength.
It is therefore an object of the present invention to provide a magnetic powder that can produce a magnet having a high magnetic flux density and having excellent magnetizability and reliability and provide an isotropic bonded magnet formed from the magnetic powder.
In order to achieve the above object, the present invention is directed to a magnet powder having an alloy composition represented by Rx(Fe1xe2x88x92yCoy)100xe2x88x92xxe2x88x92zxe2x88x92wBzAlw (where R is at least one kind of rare-earth element, x is 8.1-9.4 at %, y is 0-0.30, z is 4.6-6.8 at %, and w is 0.02-0.8 at %), and a structure in which a soft magnetic phase and a hard magnetic phase exist adjacent with each other, wherein the magnet powder has characteristics in which, when an isotropic bonded magnet is molded by mixing the magnet powder with a binding resin, the magnetic flux density (B) of the bonded magnet, in the region of B higher than the straight line for Pc (permeance coefficient)=2.0 in the second quadrant of the B-H diagram representing the magnetic characteristics at room temperature, is found always on the upper side of the straight line representing Equation (I) below, and the intrinsic coercive force (iHc) of the magnet is in the range of 5.1-9.0 kOe:
B=1.25xc3x97xcfx81+1.25xc3x97Hxe2x80x83xe2x80x83(I)
where B is the magnetic flux density, xcfx81 is the density of the bonded magnet, and H is the magnetic field.
In the above magnet powder of the present invention, high magnetic flux density can be secured. Therefore, it is possible to obtain a bonded magnet with high magnetic performance even if it is isotropic. In particular, since magnetic performance equivalent to or better than the conventional isotropic bonded magnet can be obtained with a magnet of smaller volume as compared with the conventional isotropic bonded magnet, it is possible to provide a high performance motor of a smaller size.
Further, since a higher magnetic flux density can be secured, in manufacturing a bonded magnet sufficiently high magnetic performance is obtainable without pursuing a means for elevating the density of the bonded magnet. As a result, enhancement of the dimensional accuracy, mechanical strength, corrosion resistance, thermal resistance and the like can be attained along with the improvement in the moldability, so that it is possible to readily manufacture a bonded magnet with high reliability.
Furthermore, since the magnetizability of the magnet according to this invention is excellent, it is possible to magnetize a magnet with a lower magnetizing field. In particular, multipolar magnetization or the like can be accomplished easily and surely, and further a high magnetic flux density can be obtained.
Since a high density is not required to the bonded magnet, the present invention is adapted to the manufacture of the bonded magnet by the extrusion molding method or the injection molding method by which molding at high density is difficult as compared with the compression molding method, and the effects described in the above can also be realized in the bonded magnet manufactured by these molding methods. Accordingly, the latitude of selection of the molding method and the shape for the bonded magnet can be expanded.
In the magnet powder described above, it is preferred that said structure is a nanocomposite structure in which the soft magnetic phase and the hard magnetic phase exist adjacent with each other. In this case, it is also preferred that said R comprises rare-earth elements mainly containing Nd and/or Pr. Further, said R may include Pr and its ratio to the total mass of said R is 5-75%. Furthermore, said R may include Dy and its ratio to the total mass of said R is equal to or less than 10%.
Further, in the present invention, it is also preferred that the magnet powder is obtained by quenching a molten alloy. In this case, the magnet powder is obtained by pulverizing a quenched ribbon manufactured by using a cooling roll.
Furthermore, in the present invention, it is preferred that the magnet powder is subjected to a heat treatment for at least once during the manufacturing process or after its manufacture.
Moreover, it is also preferred that the average grain size of the magnet powder lies in the range of 0.5-150 xcexcm.
The present invention is also directed to an isotropic rare-earth bonded magnet formed by binding magnet powder described above with a binding resin.
Another aspect of the present invention is also directed to an isotropic rare-earth bonded magnet formed by binding a magnet powder with a binding resin, wherein the isotropic rare-earth bonded magnet is characterized in that in the region with the magnetic flux density (B) higher than that represented by a straight line Pc (permeance coefficient)=2.0 in the second quadrant of the B-H diagram representing the magnetic characteristics at room temperature, the magnetic flux density of the bonded rare-earth magnet is always found on the upper side of the straight line representing Equation (I) below, and its intrinsic coercive force (iHc) lies in the range of 5.1-9.0 kOe:
B=1.25xc3x97xcfx81+1.25xc3x97Hxe2x80x83xe2x80x83(I)
where B is the magnetic flux density, p is the density of the bonded magnet, and H is the magnetic field.
In this isotropic bonded magnet, it is preferred that said magnet powder has a structure in which a soft magnetic phase and a hard magnetic phase exist adjacent with each other. In this case, it is also preferred that said structure is a nanocomposite structure in which the soft magnetic phase and the hard magnetic phase exist adjacent with each other.
Further, it is preferred that the bonded magnet is one to be served for multipolar magnetization or is one already subjected to multipolar magnetization.
Preferably, the isotropic bonded magnet as described is used for a motor.
These and other objects, structures and advantages of the present invention will be apparent from the following detailed description of the invention and the examples taken in conjunction with the appended drawings.